Apparatus and methods of adjusting atrioventricular pacing delay intervals in a rate adaptive pacemaker

ABSTRACT

Provided herewith are methods and apparatus for optimizing an atrioventricular (AV) pacing delay interval. One manner described involves dynamically programming an AV interval in cardiac resynchronization therapy (CRT) device having a rate-adaptive AV (RAAV) feature in such a way that not less than a minimum AV interval is maintained. That is, the AV interval is not allowed to be reduced so much that the P-wave is truncated by the QRS complex. In this form of the invention, the AV interval is reduced by one millisecond per one bpm increase in heart rate (and vice versa for reducing heart rate) but maintained at a value calculated from the end of the P-wave (PWend) and the beginning of the QRS complex (QRSbeg) or delivery of a ventricular pacing stimulus or to the end of the end of the QRS complex (QRSend).

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/047,923, filed Mar. 13, 2008 entitled “APPARATUS AND METHODS OFADJUSTING ATRIOVENTRICULAR PACING DELAY INTERVALS IN A RATE ADAPTIVEPACEMAKER”, herein incorporated by reference in its entirety. Thisapplication is related to U.S. Pat. No. 7,941,218, filed Mar. 13, 2008entitled “APPARATUS AND METHODS FOR OPTIMIZING ATRIOVENTRICULAR PACINGDELAY INTERVALS”.

FIELD

This invention relates to cardiac pacing systems, and more particularlyto apparatus and methods for improving cardiac function by optimizingatrioventricular (AV) timing intervals for a cardiac pacing system, suchas a dual-chamber or a triple-chamber cardiac resynchronization therapy(CRT) delivery system.

BACKGROUND

Cardiac resynchronization therapy (CRT) is a promising and accepteddevice therapy for patients with systolic heart failure classified inNew York Heart Association (NYHA) class III and IV. Indications includepatients who, despite optimal medication, are symptomatic, and whodemonstrate LV asynchrony. The latter occurs in patients with leftbundle branch block (LBBB) and typically presents with a QRS width(measured on an ECG machine) of greater than about 130-150 ms. Herein,“asynchrony” is characterized by a delay in systolic contraction betweenthe intraventricular septum and the left ventricular (LV) free wall.

Currently available CRT bi-ventricular pacing generally employs one leadpositioned in operative communication with the right ventricle (RV) andone lead in operative communication with a portion of one of thetributaries of the coronary venous system. The myocardial venous systemprovides a pathway for deployment of LV stimulation of the lead (andassociated electrodes) to operatively communicate with the LV. In mostpatients, an additional lead is deployed to the right atrium (RA) foratrioventricular (AV) synchronization during pacing. Exceptions forplacement of the atrial lead include patients suffering from chronicatrial fibrillation (AF) or having a relatively high AF “burden.”According to such CRT delivery, electrical stimulation of both the RVand LV operates to assist ventricular asynchrony and increasecontractility (as measured by ventricular pressure development (dP/dt).

CRT has been established as an effective treatment for heart failurepatients (NYHA III, IV) with long QRS duration (QRSd>120 ms) and lowejection fraction (EF<35%). A number of acute studies demonstrated asignificant dependence of various indices of cardiac function on theprogrammed values of the atrio-ventricular (AV) and inter-ventricular(W) delays. The most commonly used methods of AV and VV delay intervaloptimization are based on echocardiographic evaluation of fillingcharacteristics, cardiac output (CO), and ventricular dyssynchrony fordifferent interval settings. A few chronic studies demonstrated limitedevidence of long-term benefit of echo-guided interval optimization.However, considering supposedly incremental benefit of intervaloptimization such methods seem to be too time and resource-consuming.For certain patients, further optimization of the AV interval can beperformed on the guidance of echocardiographic or hemodynamic parametersas is known in the art. However, such methods of optimization of theprogrammed AV delays in triple chamber (e.g., CRT delivery) implantablemedical devices (IMDs) involve complexities. With supposedly incrementalbenefit of optimization, echocardiographic evaluation simply takes toomuch time and effort for clinicians (and clinics) and requirescoordination between implanting physicians and imaging personnel andequipment. Besides the time, effort and coordination required, thepatient is typically lying down and essentially stationary during theprocedure. Accordingly, the patient's hemodynamic state duringoptimization simply does not correlate to the state during activities ofdaily living (ADL); this is, when the patient is ambulatory.

Thus, there is a need in the art for an improved, easily optimizedpacing therapy delivery system that does not need take the above-notedfactors into consideration while preserving the benefits of the pacingsystem described above. Specifically, there is a need for apparatus andmethods to easily and efficiently control AV intervals in arate-adaptive cardiac pacing therapy delivery device (e.g., adual-chamber or a triple-chamber).

SUMMARY

Embodiments of the invention provide apparatus and methods of limitingrate-adaptive AV (RAAV) timing changes to preserve hemodynamics based onintracardiac electrograms, subcutaneous, or surface ECG. Such apparatusand methods are highly desirable as a simple and effective means ofassuring optimized pacing or CRT delivery. Certain embodiments providecomplex echocardiographic interval optimization by being based onsurface, subcutaneous (so-called leadless arrays providing electrodevectors) ECG and/or intracardiac EGM signals and can be performedacutely and chronically.

In one embodiment, the AV interval is limited in how much it can bedecreased for a patient having a high heart rate excursion. That is, theseparation of the P-wave and the accompanying QRS complex is preserved.So in a pacing device have the RAAV programmed “on” the time intervalbetween the end of the P-wave (PWend) and the beginning of the QRScomplex (QRSbeg or Q-wave) or end of complex (QRSend) is no less than acertain fixed value.

To practice the foregoing dynamically with an ambulatory subject, thetermination (PWend) or duration of the P-wave (PWd) is determinedautomatically with an novel algorithm. The algorithm begins operatingwhen an atrial event is sensed (intrinsic or paced), which can be afar-field signal sensed from electrodes spaced from the heart or nearfield electrodes within or about the heart. The sensed signal is thenfiltered and the time derivative of the P-wave (dPW/dt) is taken andrectified resulting in a waveform having, for example, two peaks. Inthis case either of the rectified peaks can be used as a reference asthe algorithm proceeds. A nominal threshold is set based at least inpart on the peak signals (P1, P2, P3, etc), such as about 15% or about30% (or other effective value) of the amplitude thereof, and a temporalwindow having a nominal length (e.g., 5, 8, 10, 12 ms) is translatedfrom one of the peaks. When the rectified signal within the temporalwindow is completely below the threshold, the end of PWend is declaredand optionally, PWd can be calculated (beginning with a sensed atrialevent, As, or a paced atrial event Ap). In a related aspect, PWd aloneprovides an indication of a subject's susceptibility to atrialarrhythmias such as atrial flutter and atrial fibrillation (collectivelyAF). In this aspect, an acute lengthening of the PWd as well as achronic lengthening, or a trend of increasing PWd values is used as adiagnostic for a subject and his or her attending clinic and clinicians.Inversely, an acute shortening or a trend of increasing shorter PWdvalues can indicate that a palliative or curative therapy regime ishaving a positive effect upon the subject or the subject is otherwisepossibly improving their cardiac condition or stability.

Of course, the algorithm is not the exclusive manner of determiningPWend and PWd, however, as other methods could be utilized such as bydetermining changes to the slope of the P-wave signal, thresholdcrossings, changing polarity of the signal, and the like.

Thus, the inventors discovered an aspect of optimizing AV intervals, andlimiting adjustment of RAAV interval that allows tuning and limiting theautomatic adjustment of the AV interval. Since RAAV intervals increasewhen the heart rate (HR) increased it can adjust away from an optimal AVinterval delay. In this example any arbitrary technique for setting oroptimizing an AV interval can be utilized but, as noted above, arate-adaptive AV (RAAV) feature must be programmed “on.” According tothis embodiment, a minimum AV is implemented so that the RAAV featuredoesn't decrease an operative AV interval too much to the detriment ofventricular filling and the patient's hemodynamic status. That is, theRAAV feature is programmed to allow the AV interval to shorten but somuch that the AV interval is too short thereby causing less than optimal(left) ventricular filling. The minimum AV interval can be based off ofeither the beginning of end of the QRS complex —QRSbeg or QRSend. Anequation for each form of these two examples appears immediately below:

AVopt−(PWend−QRSbeg−εms)

and

AVopt−(PWend−QRSend−εms)

where ε is a fixed value, such as 40 ms or other optimal value for agiven patient that promotes ventricular filling. An aspect of thisembodiment relates to the amount the sensed-AV (SAV) interval and thepaced-AV (PAV) are changed when the heart rate experiences an upwardexcursion, one millisecond (ms) for every one beat per minute (bpm)increase in the heart rate above a baseline (or recently measured RRintervals over a series of cardiac cycles) or from beat-to-beat and thelike. Inversely, if the heart rate decreases from its recent upwardexcursion the AV interval can be increased gradually back to itsprevious AVopt or other AV interval value. For example, for each one bpmdecrease in the heart rate the AV interval (SAV and PAV) can bedecreased one ms.

Cardiac activity may be sensed with a far-field sensing system; such asa shroud or surround-type subcutaneous electrode array (SEA), such asthat disclosed and depicted in co-pending application Ser. No.11/687,465 filed 16 Mar. 2007, the contents of which are incorporatedherein by re/ference. The inventors note that so-called far fieldelectrode vectors, such as via a SEA or a coil-to-can vector, oftenproduce less noise than near field (e.g., tip-to-ring) vectors althougha variety of different vectors can be tested and compared for the onethat best senses P-waves. Of course, any temporary or chronicallyimplantable medical electrical lead can be used to sense cardiacactivity (e.g., intracardiac, transvenous, and/or epicardial electrodes)deployed about the heart and used to define appropriate sensing vectorsto capture the signals (esp. P-waves) from the cardiac activity. Asnoted above, surface electrodes coupled to a display or to an IMDprogramming device can also be used. Such electrodes can be coupled to amedical device programmer or ECG machine each optionally having hardprint capability and/or a display. Currently available programmingdevices and ECG equipment can be utilized. Although exemplaryprogrammers, among others, include U.S. Pat. No. 7,209,790 entitledMulti-mode Programmer for Medical Device Communication and U.S. Pat. No.6,931,279 entitled Method and Apparatus for Implementing Task-orientedInduction Capabilities in an Implantable Cardioverter Defibrillator andProgrammer, the contents of which are incorporated herein by reference.

The foregoing and other aspects and features will be more readilyunderstood from the following detailed description of the embodimentsthereof, when considered in conjunction with the drawings, in which likereference numerals indicate similar structures throughout the severalviews.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pair of illustrations of a cardiac depolarization andrepolarization waveforms with the PQRST complex indicated by acorresponding letter and the P-wave duration (herein PWd), PR interval,QRS duration (QRSd) and QT interval of a normal intrinsic electricalactivation sequence.

FIG. 2 is a schematic diagram depicting a three channel, atrial andbi-ventricular, pacing system.

FIG. 3 is a simplified block diagram of one embodiment of IPG circuitryand associated leads employed in the system of FIG. 2 for providingthree sensing channels and corresponding pacing channels that functionsto provide therapy to and/or monitor a subject.

FIG. 4 is an elevational side view depicting an exemplary far-fieldshroud assembly coupled to an IMD, which illustrates electricalconductors disposed in the header, or connector, portion of the IMD thatare configured to couple to end portions of medical electrical leads aswell as couple to operative circuitry within the IMD housing.

FIG. 5 is a perspective view of the IMD depicted in FIG. 4 furtherillustrating the shroud assembly and two of the three electrodes.

FIG. 6 is a perspective view of an opposing major side 10″ of the IMD 10depicted in FIGS. 4 and 5 and three self-healing grommets 21substantially hermetically coupled to openings of a like number ofthreaded bores.

FIG. 7A is paired depictions of a (too) short AV interval wherein thelower is a doppler echocardiographic image of mitral flow resulting fromthe AV intervals.

FIG. 7B is paired depictions of a recommended AV interval wherein thelower is a doppler echocardiographic image of mitral flow resulting fromthe AV interval.

FIG. 7C is paired depictions of a (too) long AV interval wherein thelower is a doppler echocardiographic image of mitral flow resulting fromthe AV interval.

FIG. 8A depicts signals from a pair of surface electrodes (lead I andII) wherein a paced-AV (PAV) interval extends 130 ms (“short AV”)showing how the relative location of the P-wave and the beginning of theQRS complex relates to the PAV interval.

FIG. 8B depict signals from a pair of surface electrodes (lead I and II)wherein a paced-AV (PAV) interval extends to 170 ms (adjusted AV)showing how the relative location of the P-wave and the beginning of theQRS complex relates to the PAV interval.

FIG. 8C depict signals from a pair of surface electrodes (lead I and II)wherein a paced-AV (PAV) interval extends to 220 ms (long AV) showinghow the relative location of the P-wave and the beginning of the QRScomplex relative to the PAV interval.

FIG. 9A are paired images illustrating an embodiment wherein anoptimized AV interval, AVopt, is longer when the patient is sitting.

FIG. 9B are paired images illustrating an embodiment wherein anoptimized AV interval, AVopt, is shortened during increased heart rateexcursion such as when the patient moves from a sitting position to astanding position.

FIG. 9C are paired images illustrating an embodiment wherein anoptimized AV interval, AVopt, is shortened during increased heart rateexcursion and then returned to the AVopt interval following theincreased heart rate excursion.

FIG. 10A is a flow chart illustrating an embodiment for measuring theend of a P-wave (PWend) and/or the duration of a P-wave (PWd).

FIG. 10B is a depiction of a portion of the process depicted in FIG.10A.

FIG. 11 is a flow chart illustrating a method of calculating the linearrelationship between an optimal atrioventricular interval (AVopt) andPWd, QRS complex duration (QRSd), intrinsic P-R interval (PR), and heartrate (HR) so that chronic, dynamic control of the AVopt interval can berealized via the linear relationship or via a look up table (LUT).

FIG. 12 is a flow chart depicting an embodiment wherein a rate adaptiveatrio-ventricular (RAAV) interval is dynamically adjusted during anincreasing heart rate excursion, subject to a lower limit for a subjectreceiving cardiac resynchronization therapy.

FIG. 13 is a flow chart illustrating a diagnostic and monitoring methodfor measuring P-wave duration, P-wave end-time, and the duration of theQRS complex (QRSd) to provide notifications, alarm, and/or interventionin the event that one or more of the values acutely or chronicallychanges from historical values.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

A Methods and apparatus are provided to optimize timing intervals forand/or monitor a subject receiving cardiac resynchronization therapy(CRT) to improve the hemodynamics of the subject to closely mimic anormal depolarization-repolarization cardiac cycle sequence.

FIG. 1 is a pair of illustrations of a cardiac depolarization andrepolarization waveforms with the PQRST complex indicated by acorresponding letter and the P-wave duration (herein PWd) indicated atreference numeral 1, PR interval 2, QRS duration (QRSd) 3 and QTinterval 4 of a normal intrinsic electrical activation sequence.

FIG. 2 is a schematic representation of an implanted, triple-chambercardiac pacemaker comprising a pacemaker IPG 14 and associated leads 16,32 and 52. The pacemaker IPG 14 is implanted subcutaneously in apatient's body between the skin and the ribs. The three endocardialleads 16, 32, 52 operatively couple the IPG 14 with the RA, the RV andthe LV, respectively. Each lead includes at least one electricalconductor and pace/sense electrode, and a remote indifferent canelectrode 20 is formed as part of the outer surface of the housing ofthe IPG 14. As described further below, the pace/sense electrodes andthe remote indifferent can electrode 20 (IND_CAN electrode) can beselectively employed to provide a number of unipolar and bipolarpace/sense electrode combinations for pacing and sensing functions,particularly sensing far field signals (e.g. far field R-waves). Thedepicted positions in or about the right and left heart chambers arealso merely exemplary. Moreover other leads and pace/sense electrodesmay be used instead of the depicted leads and pace/sense electrodes thatare adapted to be placed at electrode sites on or in or relative to theRA, LA, RV and LV. In addition, mechanical and/or metabolic sensors canbe deployed independent of, or in tandem with, one or more of thedepicted leads.

The depicted bipolar endocardial RA lead 16 is passed through a veininto the RA chamber of the heart 10, and the distal end of the RA lead16 is attached to the RA wall by an attachment mechanism 17. The bipolarendocardial RA lead 16 is formed with an in-line connector 13 fittinginto a bipolar bore of IPG connector block 12 that is coupled to a pairof electrically insulated conductors within lead body 15 and connectedwith distal tip RA pace/sense electrode 19 and proximal ring RApace/sense electrode 21. Delivery of atrial pace pulses and sensing ofatrial sense events is effected between the distal tip RA pace/senseelectrode 19 and proximal ring RA pace/sense electrode 21, wherein theproximal ring RA pace/sense electrode 21 functions as an indifferentelectrode (IND_RA). Alternatively, a unipolar endocardial RA lead couldbe substituted for the depicted bipolar endocardial RA lead 16 and beemployed with the IND_CAN electrode 20. Or, one of the distal tip RApace/sense electrode 19 and proximal ring RA pace/sense electrode 21 canbe employed with the IND_CAN electrode 20 for unipolar pacing and/orsensing.

Bipolar, endocardial RV lead 32 is passed through the vein and the RAchamber of the heart 10 and into the RV where its distal ring and tip RVpace/sense electrodes 38 and 40 are fixed in place in the apex by aconventional distal attachment mechanism 41. The RV lead 32 is formedwith an in-line connector 34 fitting into a bipolar bore of IPGconnector block 12 that is coupled to a pair of electrically insulatedconductors within lead body 36 and connected with distal tip RVpace/sense electrode 40 and proximal ring RV pace/sense electrode 38,wherein the proximal ring RV pace/sense electrode 38 functions as anindifferent electrode (IND_RV). Alternatively, a unipolar endocardial RVlead could be substituted for the depicted bipolar endocardial RV lead32 and be employed with the IND_CAN electrode 20. Or, one of the distaltip RV pace/sense electrode 40 and proximal ring RV pace/sense electrode38 can be employed with the IND_CAN electrode 20 for unipolar pacingand/or sensing.

In this illustrated embodiment, a bipolar, endocardial coronary sinus(CS) lead 52 is passed through a vein and the RA chamber of the heart10, into the coronary sinus and then inferiorly in a branching vessel ofthe great cardiac vein to extend the proximal and distal LV CSpace/sense electrodes 48 and 50 alongside the LV chamber. The distal endof such a CS lead is advanced through the superior vena cava, the rightatrium, the ostium of the coronary sinus, the coronary sinus, and into acoronary vein descending from the coronary sinus, such as the lateral orposteriolateral vein.

In a four chamber or channel embodiment, LV CS lead 52 bears proximal LACS pace/sense electrodes 28 and 30 positioned along the CS lead body tolie in the larger diameter CS adjacent the LA. Typically, LV CS leadsand LA CS leads do not employ any fixation mechanism and instead rely onthe close confinement within these vessels to maintain the pace/senseelectrode or electrodes at a desired site. The LV CS lead 52 is formedwith a multiple conductor lead body 56 coupled at the proximal endconnector 54 fitting into a bore of IPG connector block 12. A smalldiameter lead body 56 is selected in order to lodge the distal LV CSpace/sense electrode 50 deeply in a vein branching inferiorly from thegreat vein GV.

In this embodiment, the CS lead body 56 would encase four electricallyinsulated lead conductors extending proximally from the more proximal LACS pace/sense electrode(s) and terminating in a dual bipolar connector54. The LV CS lead body would be smaller between the LA CS pace/senseelectrodes 28 and 30 and the LV CS pace/sense electrodes 48 and 50. Itwill be understood that LV CS lead 52 could bear a single LA CSpace/sense electrode 28 and/or a single LV CS pace/sense electrode 50that are paired with the IND_CAN electrode 20 or the ring electrodes 21and 38, respectively for pacing and sensing in the LA and LV,respectively.

Further, FIG. 3 depicts bipolar RA lead 16, bipolar RV lead 32, andbipolar LV CS lead 52 without the LA CS pace/sense electrodes 28 and 30coupled with an IPG circuit 300 having programmable modes and parametersof a bi-ventricular DDDR type known in the pacing art. In addition, atleast one physiologic sensor 41 is depicted operatively coupled to aportion of myocardium and electrically coupled to a sensor signalprocessing circuit 43. In turn the sensor signal processing circuit 43indirectly couples to the timing circuit 330 and via bus 306 tomicrocomputer circuitry 302. The IPG circuit 300 is illustrated in afunctional block diagram divided generally into a microcomputer circuit302 and a pacing circuit 320. The pacing circuit 320 includes thedigital controller/timer circuit 330, the output amplifiers circuit 340,the sense amplifiers circuit 360, the RF telemetry transceiver 322, theactivity sensor circuit 322 as well as a number of other circuits andcomponents described below.

Crystal oscillator circuit 338 provides the basic timing clock for thepacing circuit 320, while battery 318 provides power. Power-on-resetcircuit 336 responds to initial connection of the circuit to the batteryfor defining an initial operating condition and similarly, resets theoperative state of the device in response to detection of a low batterycondition. Reference mode circuit 326 generates stable voltage referenceand currents for the analog circuits within the pacing circuit 320,while analog to digital converter ADC and multiplexer circuit 328digitizes analog signals and voltage to provide real time telemetry if acardiac signals from sense amplifiers 360, for uplink transmission viaRF transmitter and receiver circuit 332. Voltage reference and biascircuit 326, ADC and multiplexer 328, power-on-reset circuit 336 andcrystal oscillator circuit 338 may correspond to any of those presentlyused in current marketed implantable cardiac pacemakers.

If the IPG is programmed to a rate responsive mode, the signals outputby one or more physiologic sensor are employed as a rate controlparameter (RCP) to derive a physiologic escape interval. For example,the escape interval is adjusted proportionally the patient's activitylevel developed in the patient activity sensor (PAS) circuit 322 in thedepicted, exemplary IPG circuit 300. The patient activity sensor 316 iscoupled to the IPG housing and may take the form of a piezoelectriccrystal transducer as is well known in the art and its output signal isprocessed and used as the RCP. Sensor 316 generates electrical signalsin response to sensed physical activity that are processed by activitycircuit 322 and provided to digital controller/timer circuit 330.Activity circuit 332 and associated sensor 316 may correspond to thecircuitry disclosed in U.S. Pat. Nos. 5,052,388 and 4,428,378.Similarly, embodiments of this invention may be practiced in conjunctionwith alternate types of sensors such as oxygenation sensors, pressuresensors, pH sensors and respiration sensors, all well known for use inproviding rate responsive pacing capabilities. Alternately, QT time maybe used as the rate indicating parameter, in which case no extra sensoris required. Similarly, embodiments of the invention may also bepracticed in non-rate responsive pacemakers.

Data transmission to and from the external programmer is accomplished bymeans of the telemetry antenna 334 and an associated RF transmitter andreceiver 332, which serves both to demodulate received downlinktelemetry and to transmit uplink telemetry. Uplink telemetrycapabilities will typically include the ability to transmit storeddigital information, e.g. operating modes and parameters, EGMhistograms, and other events, as well as real time EGMs of atrial and/orventricular electrical activity and Marker Channel pulses indicating theoccurrence of sensed and paced depolarizations in the atrium andventricle, as are well known in the pacing art.

Microcomputer 302 contains a microprocessor 304 and associated systemclock 308 and on-processor RAM and ROM chips 310 and 312, respectively.In addition, microcomputer circuit 302 includes a separate RAM/ROM chip314 to provide additional memory capacity. Microprocessor 304 normallyoperates in a reduced power consumption mode and is interrupt driven.Microprocessor 304 is awakened in response to defined interrupt events,which may include A-TRIG, RV-TRIG, LV-TRIG signals generated by timersin digital timer/controller circuit 330 and A-EVENT, RV-EVENT, andLV-EVENT signals generated by sense amplifiers circuit 360, amongothers. The specific values of the intervals and delays timed out bydigital controller/timer circuit 330 are controlled by the microcomputercircuit 302 by means of data and control bus 306 from programmed-inparameter values and operating modes. In addition, if programmed tooperate as a rate responsive pacemaker, a timed interrupt, e.g., everycycle or every two seconds, may be provided in order to allow themicroprocessor to analyze the activity sensor data and update the basicA-A, V-A, or V-V interval, as applicable. In addition, themicroprocessor 304 may also serve to define variable AV delays and theuni-ventricular, pre-excitation pacing delay intervals (A-LVp) from theactivity sensor data, metabolic sensor(s) and/or mechanical sensor(s).

In one embodiment, microprocessor 304 is a custom microprocessor adaptedto fetch and execute instructions stored in RAM/ROM unit 314 in aconventional manner. It is contemplated, however, that otherimplementations may be suitable. For example, an off-the-shelf,commercially available microprocessor or microcontroller, or customapplication-specific, hardwired logic, or state-machine type circuit mayperform the functions of microprocessor 304.

Digital controller/timer circuit 330 operates under the general controlof the microcomputer 302 to control timing and other functions withinthe pacing circuit 320 and includes a set of timing and associated logiccircuits of which certain ones pertinent to this discussion aredepicted. The depicted timing circuits include URI/LRI timers 364, V-Vdelay timer 366, intrinsic interval timers 368 for timing elapsedV-EVENT to V-EVENT intervals or V-EVENT to A-EVENT intervals or the V-Vconduction interval, escape interval timers 370 for timing A-A, V-A,and/or V-V pacing escape intervals, an AV delay interval timer 372 fortiming the A-LVp delay (or A-RVp delay) from a preceding A-EVENT orA-TRIG, a post-ventricular timer 374 for timing post-ventricular timeperiods, and a date/time clock 376.

The AV delay interval timer 372 is loaded with an operating AV intervalfor the ventricular chamber to time-out starting from a preceding atrialevent whether paced or intrinsic in nature, herein Ap and As,respectively. In one form, the AV interval is set to a nominal valuesuch as 40 milliseconds as measured from the detected end of the P-waveto the beginning of the QRS complex.

Another embodiment involves adjustment of the time interval between theend of P-wave (PWend) and the end of paced QRS (QRSend) to a fixedpredetermined value (e.g. 150 ms). In the most generic embodiment ofECG-based optimization the optimal AV delay is calculated as a linearfunction of baseline P-wave duration (PWd), baseline PR (intrinsic)interval, baseline or paced QRS duration (QRSd) and heart rate (HR):

AVopt=a*PWd+b*QRSd+c*PR+d*HR+f;

In addition the inventors discovered that the heart rate (HR) has aneffect upon value of an optimal AV delay. If Rate-Adaptive AV (RAAV)feature is programmed on, the minimum AV in the RAAV feature should beprogrammed to AVopt−(PWend−to−QRSbeg/end−ε ms) where ε is a fixed value,such as a nominal 40 ms. RAAV should then be programmed as to decreasethe sensed AV (SAV) and paced-AV (PAV) by one ms for every one beat perminute (bpm) increase in the HR.

The post-event timers 374 time out the post-ventricular time periodsfollowing an RV-EVENT or LV-EVENT or a RV-TRIG or LV-TRIG andpost-atrial time periods following an A-EVENT or A-TRIG. The durationsof the post-event time periods may also be selected as programmableparameters stored in the microcomputer 302. The post-ventricular timeperiods include the PVARP, a post-atrial ventricular blanking period(PAVBP), a ventricular blanking period (VBP), and a ventricularrefractory period (VRP). The post-atrial time periods include an atrialrefractory period (ARP) during which an A-EVENT is ignored for thepurpose of resetting any AV delay, and an atrial blanking period (ABP)during which atrial sensing is disabled. It should be noted that thestarting of the post-atrial time periods and the AV delays can becommenced substantially simultaneously with the start or end of eachA-EVENT or A-TRIG or, in the latter case, upon the end of the A-PACEthat may follow the A-TRIG. Similarly, the starting of thepost-ventricular time periods and the V-A escape interval can becommenced substantially simultaneously with the start or end of theV-EVENT or V-TRIG or, in the latter case, upon the end of the V-PACEthat may follow the V-TRIG. The microprocessor 304 also optionallycalculates AV delays, post-ventricular time periods, and post-atrialtime periods that vary with the sensor based escape interval establishedin response to the RCP(s) and/or with the intrinsic atrial rate.

The output amplifiers circuit 340 contains a RA pace pulse generator(and a LA pace pulse generator if LA pacing is provided), a RV pacepulse generator, and a LV pace pulse generator or corresponding to anyof those presently employed in commercially marketed cardiac pacemakersproviding atrial and ventricular pacing. In order to trigger generationof an RV-PACE or LV-PACE pulse, digital controller/timer circuit 330generates the RV-TRIG signal at the time-out of the A-RVp delay or theLV-TRIG at the time-out of the A-LVp delay provided by AV delay intervaltimer 372 (or the V-V delay timer 366). Similarly, digitalcontroller/timer circuit 330 generates an RA-TRIG signal that triggersoutput of an RA-PACE pulse (or an LA-TRIG signal that triggers output ofan LA-PACE pulse, if provided) at the end of the V-A escape intervaltimed by escape interval timers 370.

The output amplifiers circuit 340 includes switching circuits forcoupling selected pace electrode pairs from among the lead conductorsand the IND_CAN electrode 20 to the RA pace pulse generator (and LA pacepulse generator if provided), RV pace pulse generator and LV pace pulsegenerator. Pace/sense electrode pair selection and control circuit 350selects lead conductors and associated pace electrode pairs to becoupled with the atrial and ventricular output amplifiers within outputamplifiers circuit 340 for accomplishing RA, LA, RV and LV pacing.

The sense amplifiers circuit 360 contains sense amplifiers correspondingto any of those presently employed in contemporary cardiac pacemakersfor atrial and ventricular pacing and sensing. As noted in theabove-referenced, commonly assigned, '324 patent, a very high impedanceP-wave and R-wave sense amplifiers may be used to amplify the voltagedifference signal that is generated across the sense electrode pairs bythe passage of cardiac depolarization wavefronts. The high impedancesense amplifiers use high gain to amplify the low amplitude signals andrely on pass band filters, time domain filtering and amplitude thresholdcomparison to discriminate a P-wave or R-wave from background electricalnoise. Digital controller/timer circuit 330 controls sensitivitysettings of the atrial and ventricular sense amplifiers 360.

The sense amplifiers are uncoupled from the sense electrodes during theblanking periods before, during, and after delivery of a pace pulse toany of the pace electrodes of the pacing system to avoid saturation ofthe sense amplifiers. The sense amplifiers circuit 360 includes blankingcircuits for uncoupling the selected pairs of the lead conductors andthe IND_CAN electrode 20 from the inputs of the RA sense amplifier (andLA sense amplifier if provided), RV sense amplifier and LV senseamplifier during the ABP, PVABP and VBP. The sense amplifiers circuit360 also includes switching circuits for coupling selected senseelectrode lead conductors and the IND_CAN electrode 20 to the RA senseamplifier (and LA sense amplifier if provided), RV sense amplifier andLV sense amplifier. Again, sense electrode selection and control circuit350 selects conductors and associated sense electrode pairs to becoupled with the atrial and ventricular sense amplifiers within theoutput amplifiers circuit 340 and sense amplifiers circuit 360 foraccomplishing RA, LA, RV and LV sensing along desired unipolar andbipolar sensing vectors.

Right atrial depolarizations or P-waves in the RA-SENSE signal that aresensed by the RA sense amplifier result in a RA-EVENT signal that iscommunicated to the digital controller/timer circuit 330. Similarly,left atrial depolarizations or P-waves in the LA-SENSE signal that aresensed by the LA sense amplifier, if provided, result in a LA-EVENTsignal that is communicated to the digital controller/timer circuit 330.Ventricular depolarizations or R-waves in the RV-SENSE signal are sensedby a ventricular sense amplifier result in an RV-EVENT signal that iscommunicated to the digital controller/timer circuit 330. Similarly,ventricular depolarizations or R-waves in the LV-SENSE signal are sensedby a ventricular sense amplifier result in an LV-EVENT signal that iscommunicated to the digital controller/timer circuit 330. The RV-EVENT,LV-EVENT, and RA-EVENT, LA-SENSE signals may be refractory ornon-refractory, and can inadvertently be triggered by electrical noisesignals or aberrantly conducted depolarization waves rather than trueR-waves or P-waves.

Operative circuitry 300 of FIG. 3 includes RR interval comparator 301,coupled to RV sensing electrodes coupled to lead 32, LV pacingelectrodes coupled to LV pacing electrodes coupled to lead 52. In oneembodiment, an AV interval adaptation circuit 305 operates to adjust andmaintain the AV delay interval at an optimized value. The AV intervaladaptation circuit 305 may include circuitry for modifying the optimumAV interval value in the case a rate-adaptive AV feature is programmed“on” such that the interval will decrease approximately one millisecond(ms) for each one bpm a subject's heart rate increases. In anotheraspect, the circuit 305 (in conjunction with memory structures) includestracking capability so that as, for instance, the end of the P-wave(PWend) or the duration of the P-wave (PWd) changes and thus, the AVinterval varies, these values can be subsequently reviewed. One of apair of output signals from the AV interval adaptation circuit 305operatively connect to atrial sensing and pacing electrodes that arecoupled to atrial lead 16. The other of the pair of output signals fromthe AV interval adaptation circuit 305 operatively connects to LVpelectrodes coupled to pacing electrodes coupled to the lead 52.

As noted hereinabove, a subcutaneous electrode array (SEA) can be usedto sense P-waves from a location spaced from the heart. On such SEA thatcan be coupled to or incorporated into an subcutaneously implanteddevice is shown in FIG. 4 which is an elevational side view depicting anexemplary shroud assembly coupled to an IMD which illustrates electricalconductors disposed in the header, or connector, portion of the IMDwhich is configured to receive a proximal end portion of medicalelectrical leads (not shown).

FIG. 4 depicts an exemplary shroud assembly 141 coupled to an IMD 101which illustrates electrical conductors 25, 26, 28′ disposed in theheader, or connector, portion 12 of the IMD 10 which are configured tocouple to end portions of medical electrical leads as well as couple tooperative circuitry within the IMD housing (not shown). The shroudassembly 141 surrounds IMD 101 and mechanically couples to the headerportion 12 and includes at least three discrete electrodes 16, 18, 201adapted for sensing far-field, or extra-cardiac electrogram (EC-EGM)signals. FIG. 4 also depicts an aperture 22 formed within the header 12which can be used to receive thread used to suture the header 12 (andthus the IMD 101) to a fixed surgical location (also known as a pocket)of a patient's body.

As partially depicted in FIG. 4, an elongated conductor 18′ couples toelectrode 18, elongated conductor 16′ couples to electrode 16, andconductor segment 201′ couples to electrode 201. Furthermore, three ofthe conductors (denoted collectively with reference numeral 24) coupleto three cuff-type conductors 25, 26, 28′ adapted to receive proximalportions of medical electrical leads while another three of theconductors couple to conductive pads 25′, 26′, 28″ which are alignedwith, but spaced from the conductors 25, 26, 28′ along a trio of bores(not shown) formed in header 12.

FIG. 5 is a perspective view of the IMD 101 depicted in FIG. 4 furtherillustrating the shroud assembly 141 and two of the three electrodes18,201. In addition, two of a plurality of adhesive ports 31 and amechanical joint 132 between the elongated portion of the shroudassembly 141 and the header 12 are also depicted in FIG. 5. The ports 31can be used to evacuate excess medical adhesive disposed between theshroud assembly 14 and the IMD 10 and/or used to inject medical adhesiveinto one or more of the ports 31 to fill the void(s) therebetween. Inone form of the invention, a major lateral portion 12′ of header 12remains open to ambient conditions during assembly of the IMD 101.Subsequent to making electrical connections between the plurality ofconductors of the shroud assembly 141 and the header 12, the openlateral portion 12′ is sealed (e.g., automatically or manually filledwith a biocompatible substance such as a substantially clear medicaladhesive, such as Tecothane® made by Noveon, Inc. a wholly ownedsubsidiary of The Lubrizol Corporation). Thus most if not all of theplurality of conductors of the shroud assembly 141 and the IMD 101 arevisible and can be manually and/or automatically inspected to ensurelong term operability and highest quality of the completed IMD 101.

Referring again to FIG. 4, the terminal ends of conductors 24 aredepicted to include the optional shaped-end portion which provides atarget for reliable automatic and/or manual coupling (e.g., laserwelding, soldering, and the like) of the terminal end portions torespective conductive pins of a multi-polar feedthrough assembly (notshown). As is known in the art, such conductive pins hermetically coupleto operative circuitry disposed within the IMD 101.

FIG. 6 is a perspective view of an opposing major side 101′ of the IMD101 depicted in FIGS. 4 and 5 and three self-healing grommets 23substantially hermetically coupled to openings of a like number ofthreaded bores (not shown). As is known, the threaded bores areconfigured to receive a threaded shank and the grommets 23 arefabricated to temporarily admit a mechanical tool (not shown). The toolis used to connect and allow a physician or clinician to manuallytighten the conductors 25, 26, 28′, for example, with compression and/orradially around conductive rings disposed on proximal portions ofmedical electrical leads (not shown). In addition, two of the pluralityof ports 31 are also depicted in FIG. 6.

FIGS. 7A-C are paired depictions of a (too) short AV interval, arecommended AV interval, and a (too) long AV interval wherein the loweris a doppler echocardiographic image of mitral flow resulting from thedifferent AV intervals. In FIG. 7A, the ventricular pacing stimulus Vpimpinges up the P-wave (PW) thereby not allowing adequate atrial “kick”nor complete ventricular filling. In FIG. 7B, a nominal 40 ms timeinterval is maintained following the end of the P-wave (PWend), whichcan be located per the algorithm detailed at FIG. 10 hereinbelow. FIG.7C depicts an AV interval that is too long and wherein the mitral flowis inhibited due to the lack of coordination of atrial and ventricularfunction.

FIGS. 8A-C depict signals from a pair of surface electrodes (lead I andII) wherein a paced-AV (PAV) interval varies from 130 ms (“short AV”),to 170 ms (adjusted AV), to 220 ms (“long AV”) showing how the relativelocation of the P-wave (PW) and the beginning of the QRS complex (QRS)changes with differing PAV interval. In FIG. 8A, it is apparent thatwhen the PAV is set to 130 ms the PW is truncated and as depicted thetrace from the lead labeled ECG Lead I literally collides with the QRScomplex and related fluid and electromechanical activity. Also, in FIG.8A the ECG Lead II did not even pick up the PW from that particularcardiac cycle. In FIG. 8B, at a PAV of 170 ms the PW ends approximately20-40 ms before the beginning of the QRS complex and as noted in FIGS.7A-C the concomitant ventricular filling is maximized. In FIG. 8C at aPAV of 220 ms, approximately 60 ms elapses after the end of the P-Wavebut before the QRS complex commences.

FIGS. 9A-C are paired images illustrating an embodiment wherein anoptimized AV interval, AVopt, is shortened during increased heart rateexcursion and then returned to the AVopt interval following theincreased heart rate excursion. The AVopt interval is shortened to notless than a minimum value defined as AVopt less the interval between theend of the P-wave and either the beginning or end of the QRS complex(and less a nominal additional amount, such as about 40 ms). When thepatient's heart rate stabilizes then the AVopt interval can resumeoperation (or an adapted value can be utilized according to certain ofthe embodiments described hereinbelow.

FIG. 10A is a flow chart illustrating an embodiment for measuring theend of a P-wave (PWend) and/or the duration of a P-wave (PWd). FIG. 10Acan be reviewed along with the simplified illustration of FIG. 10B,which provides a depiction of a waveform being processed according tomethod 100. The method 100 begins at 102 with collection of cardiacsignals following either a sensed or paced atrial event (As or Ap). Thesensed signals are then filtered at 104 and the time derivative (dPW/dt)is taken at 106. The resulting waveform is of course sinusoidal as theP-wave is a generally smoothly rising and then falling signal. At step108 the derived sinusoidal P-wave is rectified thus resulting in adual-humped signal. A peak of this signal is located at step 110 fromeither peak (P1 or P2) and a threshold is set based at least in partupon the amplitude of either peak P1 or P2 at step 112. The thresholdcan be a nominal value but a value of about ten to thirty percent(10%-30%) of the peak amplitude of P1 or P2 will suffice. At step 114 atemporal window is scaled from either of the peaks (P1 or P2) until thevalues of the signal beneath the window are all sub-threshold (116). Atthat point the end of the P-wave (PWend) has been located and theduration of the P-wave (PWd) can be calculated (at 120) as the timeelapsed from the atrial event (As or Ap) until PWend was located. Thenoptionally, according to certain embodiments, the value of PWd can besorted and/or compared to prior PWd values, thereby providing clinicalbenefit to a subject as an indicator of cardiac status and/or condition.In addition, the duration of the QRS complex (QRSd) can be measured andcompared to prior values. Thus, a notification, alert or notation thatthe subject is either benefiting or declining status can be performed aswill be described hereinbelow.

FIG. 11 is a flow chart illustrating a method 200 of calculating thelinear relationship between an optimal atrioventricular interval (AVopt)and PWd, QRS complex duration (QRSd), intrinsic P-R interval (PR), andheart rate (HR) so that chronic, dynamic control of the AVopt intervalcan be realized via the linear relationship or via a look up table(LUT). The cardiac cycle of subject is monitored at 202 and the P-waveduration (PWd) is measured and stored at 204 as previously described.The duration of the QRS complex of the subject is measured and stored atstep 206. The intrinsic P-R interval of the subject is measured as thetime between an intrinsic atrial event (As) and a resulting intrinsicventricular event (Vs) and stored at 208. The heart rate (HR) is thenmeasured as the time between successive QRS complexes (R-R interval) andstored. Then at 212 a confirmed optimized AV interval (AVopt) isobtained, for example using convention echocardiography or other method.Then at 214 the linear relationship is calculated that relates AVopt toPWd, QRSd, PR, and HR (including coefficients). This thus provides amethod to dynamically recalculate the AVopt interval based on detectedchanges to one or more of the four values. That is, the AVopt intervalequation can be recalculated or a look up table (LUT) populated withvalues that correlate the four values. In a related embodiment, the LUTcan be simplified somewhat due to the fact that QRSd and PWd changelittle, if any, over a fairly large ranges of heart rates for mostcardiac patients receiving cardiac resynchronization therapy (CRT). So,at step 216, in the event that one or more of the values change theAVopt can be modified at 220. In the event that the values have notchanged (or have only changed slightly) at step 218 the CRT deliverycontinues at the prior value of AVopt preferably using the same gradualrate utilized to previously shorten the AV interval (e.g., one ms perone bpm that the heart rate changes).

FIG. 12 is a flow chart depicting an embodiment 400 wherein a subjectreceiving cardiac resynchronization therapy and has a rate adaptiveatrio-ventricular (RAAV) interval that is dynamically adjusted during anincreasing heart rate excursion, subject to a lower limit for a subjectreceiving cardiac resynchronization therapy. At step 402 the cardiaccycle is monitored and the heart rate metric is calculated. At 404whether or not the heart rate is increasing or not is determined. If theheart rate is determined not to be increasing then the CRT deliverycontinues. If, however, the heart rate is determined to be increasingthen at 406 the operating AV (AVopt) interval is decreased at a rate ofabout one millisecond (ms) for every one beat per minute (bpm) the heartrate increases, subject to a limit defined as the AVopt interval lessthe interval between the end of the P-wave (PWend) and either thebeginning or end of the QRS complex (QRSbeg or QRSend) less a nominalvalue such as 40 ms. In the event that the heart is trending lower thenat 410 the value of the operating AV interval is returned to the prioroptimized AV interval value (AVopt). If the heart rate is not lowering,then the cardiac cycle continues to be monitored at 402 and process 400continues.

FIG. 13 is a flow chart illustrating a diagnostic and monitoring method500 for measuring the parameters P-wave duration (PWd), P-wave end-time(PWend), and the duration of the QRS complex (QRSd) to providenotifications, alarm, and/or intervention in the event that one or moreof the values acutely or chronically changes from historical values.According to this embodiment, the parameters PWd, PWend or QRSd aremonitored at step 502 and the values are stored at step 504. The storedvalues can then be compared at 506 with prior values or evaluated todiscern if any trend is occurring in one or more of the parameters. Atstep 508 any acute changes or trend from prior or historical values isused to trigger a logical flag, provide a notification to a clinic orclinician or the like, store a recent temporal record of recentlymeasured or recorded physiologic events, set a patient alert or thelike.

Of course, certain of the above-described structures, functions andoperations of the pacing systems of the illustrated embodiments are notnecessary to practice the present invention and are included in thedescription simply for completeness of an exemplary embodiment orembodiments. It will also be understood that there may be otherstructures, functions and operations ancillary to the typical operationof an implantable pulse generator that are not disclosed and are notnecessary to the practice of the present invention.

In addition, it will be understood that specifically describedstructures, functions and operations set forth in the above-referencedpatents can be practiced in conjunction with the present invention, butthey are not essential to its practice. It is therefore to beunderstood, that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described withoutactually departing from the spirit and scope of the present invention.

1. An apparatus for adjusting rate-adaptive atrioventricular (RAAV)pacing interval in a triple chamber cardiac resynchronization therapy(CRT) delivery device, comprising: means for measuring a heart ratechange of a subject receiving pacing therapy from an implanted pacingdevice that includes an active rate-adaptive AV (RAAV) interval feature;means for determining ends of a P-wave (PWend) for an atrialdepolarization; means for determining one of a beginning of a QRScomplex (QRSbeg) and an end of the QRS complex (QRSend) accompanying theP-wave; and means for limiting adjustment of an operating AV interval bythe RAAV interval feature to maintain a lower limit on time elapsedbetween PWend and one of the ending of the QRSbeg, QRSend, and deliveryof a ventricular pacing stimulus to preserve atrial-ventricular fillingcharacteristics.
 2. An apparatus according to claim 1, furthercomprising: means for calculating a sensed-AV (SAV) interval based uponthe operating AV interval value.
 3. An apparatus according to claim 2:wherein the calculated SAV interval is decremented in the event that thechanging heart rate indicates an increasing HR.
 4. An apparatusaccording to claim 3, wherein the SAV interval is decremented betweenabout 0.25 milliseconds (ms) and about 3 ms for each additional beat perminute (bpm) the HR increases.
 5. An apparatus according to claim 1,further comprising: means for calculating a paced-AV (PAV) intervalbased upon the operating AV interval value.
 6. An apparatus according toclaim 5, wherein the calculated PAV interval is decremented in the eventthat the HR excursion indicates an increasing HR.
 7. An apparatusaccording to claim 6, wherein the PAV interval is decremented betweenabout 0.25 milliseconds (ms) and about 3 ms for each additional beat perminute (bpm) the HR increases.
 8. An apparatus according to claim 1,comprising a chronic pacing and sensing lead and wherein the heart rateexcursion is measured via the chronic pacing and sensing lead.
 9. Anapparatus of adjusting a rate-adaptive atrio-ventricular (RAAV) pacinginterval in pacing therapy delivery device, comprising: means formeasuring a heart rate change of a subject receiving pacing therapy froman implanted pacing and determining an active rate-adaptive AV (RAAV)interval; means for determining the end of a P-wave (PWend) for anatrial depolarization; and means for limiting adjustment of an operatingAV interval in response to determination of the RAAV interval tomaintain a lower limit on time elapsed between PWend and delivery of aventricular pacing stimulus.
 10. An apparatus according to claim 9,further comprising: means for calculating one of a sensed-AV (SAV)interval and a paced-AV (PAV) interval based upon the operating AVinterval value.
 11. An apparatus according to claim 10, wherein one ofthe calculated SAV and the calculated PAV interval is decremented in theevent of an increasing [HR] heart rate (HR).
 12. An apparatus accordingto claim 11, wherein one of the SAV interval and the PAV interval isdecremented between 0.25 milliseconds (ms) and 3 ms for each additionalbeat per minute (bpm) the HR increases.